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Biology A & B: Viruses, Infection & Immunity
Name: _______________________________
Date: ________________________________
Biology A & B: Viruses, Infection & Disease Processes
Viruses and Infection
Vocabulary for this topic includes:
Virus:
Host Cell:
Nucleic Acid:
Capsid:
Bacteriophage:
Lytic Cycle:
Lysogenic Cycle:
Dormant:
Viruses are not included in any of the five kingdoms of organisms because they have
some characteristics of living things and some characteristics of non-living things. Similar
to living things, viruses can move, reproduce, and respond to their environment, but unlike
living things, they are not composed of cells and cannot live or reproduce without a host
cell. Viruses are cells that need a living host cell to reproduce. A host cell is a cell of
another organism that a virus lives in or infects.
After a person comes in contact with a virus, the virus enters a cell and may either
disturb the cell's activities or reproduce and kill the cell, resulting in the virus spreading to
new cells. If the virus is not destroyed, it can continue to replicate and re-infect the person.
Some viruses that infect humans are influenza, the common cold, chicken pox, measles, and
AIDS. Viruses are composed of nucleic acid, which is genetic material, in a protein capsid
(coat), and sometimes have spikes protruding from the capsid. The following are
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Biology A & B: Viruses, Infection & Immunity
illustrations of two different types of viruses:
Though all viruses contain basically the same structures, they can have very different
shapes. Viruses are classified by their shape, size, the type of genetic material they
contain, and the type of organism they infect. Viruses that infect bacteria are called
bacteriophages.
Viruses can reproduce in two ways, through the lytic cycle or the lysogenic cycle (also
called lysogeny). The type of virus determines which cycle will occur. Both cycles begin
with the virus attaching to a host cell. Next, the virus injects the host cell with nucleic acid,
emptying its capsid. At this point, the two cycles begin to differ.
During the lytic cycle, the host cell accepts the virus's nucleic acid instead of its own and
then reproduces. New viruses form inside the cell, take over the cell, and burst out of the
membrane, killing the cell. The new viruses spread to new cells and the lytic cycle begins
again.
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Biology A & B: Viruses, Infection & Immunity
The following is an illustration of the lytic cycle:
The lysogenic cycle is slightly different. After the virus's nucleic acid is injected into
cell, the nucleic acid becomes part of the host cell's genetic material, but is dormant; the
virus does not destroy the cell. The host cell carries the nucleic acid around, reproduces,
and lives normally. Eventually, the virus comes out of its dormancy, forms new viruses
inside the cell, and they burst out, killing the cell. Since the cell was able to reproduce
before bursting, all the new cells that were produced are already infected with the viral
nucleic acid and will go through lysogeny as well.
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Biology A & B: Viruses, Infection & Immunity
The following is an illustration of the lysogenic cycle:
Comparison Chart: Lytic & Lysogenic Cycles~
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Biology A & B: Viruses, Infection & Immunity
Disease and Impact on Immune System
Vocabulary for this section includes:
Pathogens:
Bacteria:
Fungus (Fungi):
Virus:
Immune System:
Antibiotics:
Vaccine:
Immunity:
Antibodies:
The three main types of disease-causing pathogens are as follows: those caused by
bacteria, by fungi, and by viruses. A person can contract a bacterial, fungal, or viral
disease through direct contact with harmful bacteria, fungi, or viruses, and sometimes
through inhalation. It is difficult to distinguish among the three types of diseases based on
their symptoms. This is because each type can have a variety of symptoms, or they can
have similar symptoms. .
Bacteria are single-celled organisms containing genetic material, but having no true
nucleus. A nucleus is the part of a cell that contains the genetic material in most cells and
controls a cell's activities. When bacteria enter a cell, they either destroy the cell or
produce harmful chemicals inside it. Some diseases caused by bacteria include strep
throat, pneumonia, meningitis, tuberculosis, tetanus, and cholera. Antibiotics are
medicines used to stop the growth of bacteria cells.
Fungi are organisms that obtain food by absorbing nutrients from their surroundings.
Fungi usually stay on the surface of the skin and appear as rashes, but they can infect
internal organs as well. After the spores (seed-like capsules used for reproduction) of a
fungus enter the body, the fungus begins to grow, infecting or damaging the surrounding
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Biology A & B: Viruses, Infection & Immunity
tissue. Some examples of fungal diseases are athlete's foot, ringworm, and nail fungus.
Fungal infections can sometimes go away without any treatment when the immune system
is able to combat the infection on its own. Most fungal infections can be treated with antifungal creams and medications.
Viruses are cells that need a living host cell to reproduce. After a person comes in
contact with a virus, the virus enters a cell and may either disturb the cell's activities or
reproduce and kill the cell, resulting in the virus spreading to new cells. If the virus is not
destroyed, it can continue to replicate and re-infect the person. Many years after being
infected by a virus, the person may experience symptoms of the virus again. Common
viruses include influenza (the flu), chicken pox, measles, the West Nile virus, and rabies.
Many viruses are prevented with vaccines. Vaccines contain weakened or dead forms of
the virus that stimulate the immune system to build up chemicals called antibodies to fight
off the virus if the person should come in contact with it. Vaccines are given orally (by
mouth), nasally (by nose), or by injection (by a shot, usually in the upper arm).
To combat infectious diseases, a healthy immune system locates bacteria, viruses, or
fungi and tries to kill them. Antibodies work with white blood cells to destroy these foreign
substances. Often, this causes a collection of pus and swelling in the area of infection or
other symptoms, such as fever. Once antibodies are released, they stay in the body. The
immune system will be able to fight off the same disease more easily if contracted in the
future because more antibodies can be produced.